Multiple source vibrator technology has been used in land-based seismic surveys for years, and its advantages are well documented. Conventional land-based seismic assays employ multiple, simultaneously energized seismic sources (e.g., trucks with vibrating baseplates) to impart vibratory energy into the ground. The imparted vibratory energy travels through the ground, is reflected and/or refracted by various discontinuities in the ground, and the reflected vibratory energy is detected by multiple seismic receivers (e.g., geophones) that are located on the ground at a distance from the seismic sources. The reflected vibratory energy recorded by the receivers is a composite reading representing the reflected energy originating from all the seismic sources. An important step in conventional multiple source vibrator technology is “source separation” of the composite data into discrete source specific data (ie: a “shot record”).
In order to allow for source separation, conventional multiple source vibrator technology requires multiple sweeps to be performed while the seismic sources maintain a fixed location. According to conventional multiple source vibrator technology, the number of sweeps performed at a fixed source location must be equal to or greater than the number of sources sought to be separated. For example, if four seismic sources are being used in a conventional multiple source vibrator acquisition scheme, at least four sweeps must be carried out for each source set-up. In addition, conventional multiple source vibrator technology requires uniquely encoded (e.g., phase, frequency, and/or amplitude encoded) vibratory energy for each sweep, so that source separation of the resulting composite data can be performed.
Marine data are traditionally collected using multiple air-gun sources with multiple streamers of receivers. Multiple sources mean flip-flop shooting, decreased fold, aliased multiples and other undesirable acquisition patterns. Multiple streamers mean increased minimum-offset for some bins and increased noise from the diverging side wings for other bins. Single-source multi-streamer acquisitions offer much better sampling, but with such low productivity (in terms of square kilometers acquired per day) that 3D acquisition costs become prohibitive.
Multiple vibratory sources are often required to improve acquisition efficiency and to provide enough power to transmit a signal through water and into the geological structures below. Simultaneous-multiple-source-land acquisition including ZENSEIS™ Seismic has increased field-acquisition efficiency and improved spatial resolution of land-based seismic images. The conventional ZENSEIS™ Seismic acquisition technique requires multiple sources and receivers to remain at the same locations for a number of repeated sweeps. However, marine acquisition using multiple vibratory sources violates this assumption, as both sources and receivers are moving during the sweep. Since the sources and receivers do not remain in a fixed location during the complete frequency sweep the inversion formulation used in the land case for source separations simply does not work for multiple source marine seismic studies.
Multiple methods have been proposed to use vibrational sources for marine seismic studies. Landrum, U.S. Pat. No. 4,715,020, stacks multiple data and depends upon phase correlation to enhance the desired signal and cancel vibratory signals from other sources. Sallas, et al., U.S. Pat. No. 5,721,710, activate multiple vibratory sources simultaneously with encoded signals between the sources and the sweeps but require the source and receiver to be fixed. Jeffryes, U.S. Pat. No. 6,028,818, uses exact positioning to align an odd number of vibratory sources during multiple sweep cycles where at least one of the sources generate a different phase angle signal. U.S. Pat. No. 6,049,507, Allen, describes a process for correcting source motion independently from the correction for receiver motion where each data point is corrected for source/receiver motion independently. Barr and Dragoset, U.S. Pat. No. 6,161,076, use a recorded Doppler shift to correct reflected marine data. De Kok, U.S. Pat. No. 6,545,944, describes several simultaneous source separation techniques applicable to either marine or land environments including source signals coded with positive and negative polarities that are not restricted to stationary locations, and source signals with time-delays between source activation times. In U.S. Pat. No. 6,754,590, Moldoveanu uses simultaneous overlapping vibratory sources and stacking of common midpoint data to process data. Krohn and Johnson, US2006164916, simultaneously operate multiple seismic vibrators using continuous sweeps with little or no listening time between sweeps for each vibrator, and recover the separated seismic responses for each vibrator with the earth signature removed.
Proper source separation is required while both vibrator sources and receivers are moving during the sweep. Although some of the above references account for movement of the source, receiver, or both, unfortunately, none of the above references provide unique source separations. What are required are unique source separations that can be obtained in simultaneous-multiple-source marine acquisition. Individual, non-synchronized source data must be retrieved by incorporating better constraints to produce unique source separation.